This invention relates to a driving system of a step motor principally for an electronic watch. For a step motor such as ultra-small-sized one for an electronic watch, for which a low power consumption is required, a correction drive system is devised, as a method for low-power consumption, which drives with low electric power in a normal state, and does a correction drive with bigger electric power than a normal state when a rotor does not rotate normally by a certain reason, in order to improve an electro-mechanical transducing efficiency of the step motor itself.
When this correction drive system is adopted, it is important how to detect a rotation state and non-rotation state of the rotor and how to defend the rotor from stopping in outer conditions such as magnetic field.
FIG. 1 (A) shows a step motor used for an operation of an electronic timepiece and which is used in this invention, too.
FIG. 1 (B) is a sample of an inverted pulse used to drive a step motor of the conventional type.
By applying a driving pulse of FIG. 1 (B) to a coil 3, a stator 1 is magnetized and a rotor is rotated in 180.degree. by a repulsion and a attraction of a rotor 2 and magnetic poles.
In the conventional motor, a width of an impressed driving pulse is selected one which ensures an output of a motor in any condition which must be filled as a timepiece. However, in this method, a surplus for a calendar-load, the internal resistance of a battery, and a voltage-drop at the end of battery life must be provided and drive has to be carried with a pulse-width having a surplus width. For that, by improving this method, another method is proposed, by which a step motor is driven in the normal state by a pulse-width without any surplus, a detecting circuit is used which judges the rotation state and non-rotation state of the rotor and a correction drive with a correction pulse width is executed only when the rotor state is judged to be "non-rotation".
A method of judging rotation and non-rotation of a rotor by taking into account the characteristic that there is a difference in the generation of electricity between the rotation state and the non-rotation state is used because of the difficulty of mounting a detector on the outside which performs a detection of the rotation and non-rotation of this rotor because of requirements such as cost, miniaturization and a reduction of thickness of a watch.
FIG. 3 shows a current waveform of a conventional step motor. In FIG. 3, a section "a" is the time when a driving pulse is impressed, and "b" is the time when a current is generated after impression of a driving pulse owing to an induced current made by a vibration of the rotor. The waveform "b.sub.1 " shown in FIG. 3 is one in the case of rotation of the rotor, and "b.sub.2 " is a waveform when the rotor does not rotate.
A driving circuit in this case is described in FIG. 4 (A) and numerals 4 and 5 are inverters which consist of N-channel field effect transistors and P-channel field effect transistor, and a coil 3 connected to an output thereof. After being impressed with a driving pulse, the coil 3 is changed to a short condition by the transistors which compose the inverter. Then, a current flows in the time shown by "b" in the FIG. 3 by the vibration of the rotor. In FIG. 3, "b.sub.1 " is a waveform in the rotation state and b.sub.2 is a waveform in the non-rotation state, showing a voltage which is of the product of the ON-resistance of a driving transistor and the current of a vibration of the rotor.
This generated voltage is converted into a voltage in the same sense via a transmission gate TG. Then, a comparison of a base voltage and peak voltage is carried out by a voltage comparator, rotation and non-rotation of the rotor are judged and when the rotor is judged "non-rotation", a correction drive is executed. But in this method, when an electronic timepiece is placed in an alternating current magnetic field, such as an external magnetic field, it induces a voltage in a coil, and said voltage acts as an external noise which is added to a detecting signal, when rotation and non-rotation of the rotor is judged by an induced voltage made by rotor's movement.
For that reason, detection mistakes happen as do mistakes of the movement of the rotor, that is, the timepiece gets out of order. FIG. 5 is a graph of an alternating current magnetic field proof characteristic against a driving pulse-width of a step motor.
As a low electric power consumption is required for an electronic timepiece, the driving pulse-width is set forth as short as possible. The magnetic proof characteristic for this normal driving pulse is this made worse, therefore, in order to use this correction driving system for a watch and to obtain the anti-magnetic characteristic, the magnetic proof structure must be more severe than before. Although, saving of current is possible, it's necessary to add a shield plate for the magnetic proof structure, which is an abstraction for the miniaturization and causes high-cost.